This invention relates to improvements in an air valve type carburetor in which a venturi opening-area is varied in response to a vacuum level or a negative pressure prevailing in a venturi portion so that it is maintained substantially constant.
In an air valve type carburetor, a throttle valve is provided within a horizontal air intake passage of a carburetor, a suction piston housed in a suction chamber is provided upstream of the throttle valve to move so that a vacuum level is maintained substantially constant, and a jet needle having a tapered portion is secured to the bottom of the suction piston to move within a nozzle communicating with a float chamber. The air valve type carburetor has a feature that a vacuum level in the venturi portion is maintained substantially constant irrespective of changes of running conditions as above-mentioned. Precisely stated, the vacuum level prevailing in the venturi portion increases a little in proportion to elevation of the suction piston because a suction spring is compressed to increase its compression force as the suction piston elevates. However, a desired air-fuel ratio is obtained always because the jet needle is formed not is a straightly tapered shape but in a curvilinearly tapered shape in section. Thus, in a usual operation, a good air-fuel ratio is obtained, but under a transitional running condition in which a running condition is abruptly changed, the suction piston is delayed largely in operation due to its momentum and the air-fuel ratio changes largely for a while. This phenomenon can not be expected to be avoided even if a method in which the suction piston is controlled by detection of exhaust gas compositions of an engine is adopted, because about 0.5 second is necessary for the exhaust gas to reach a detector for detecting the exhaust gas compositions, so that the operation delay can not be avoided.
Therefore, apparatus or device such that even under a transitional condition, a good air-fuel ratio can be obtained is desired.
As a relevant prior art, there is Japanese Laying-open of Patent Application No. 46234 (1977) which has a similarity that a vacuum level in an upper piston chamber 10a is controlled by an electromagnetic valve 17.